Caesium oxide[1][2]
Caesium oxide
  Caesium cations, Cs+
  Oxide anions, O2−
IUPAC name
Caesium oxide
Other names
Cesium oxide (US)
3D model (JSmol)
ECHA InfoCard 100.039.693 Edit this at Wikidata
EC Number
  • 243-679-0
  • InChI=1S/2Cs.O/q2*+1;-2 checkY
  • InChI=1/2Cs.O/q2*+1;-2
  • [Cs+].[Cs+].[O-2]
Molar mass 281.810 g·mol−1
Appearance Yellow-orange solid
Density 4.65 g/cm3, solid
Melting point 490 °C (914 °F; 763 K) (under N2)
Reacts to form CsOH
1534.0·10−6 cm3/mol
anti-CdCl2 (hexagonal)
76.0 J/(K·mol)
146.9 J/(K·mol)
−345.8 kJ/mol
Occupational safety and health (OHS/OSH):
Main hazards
GHS labelling:
GHS05: CorrosiveGHS08: Health hazard
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
Flash point non-flammable
Related compounds
Other anions
Other cations
Related caesium oxides
Related compounds
Caesium hydroxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Caesium monoxide or caesium oxide is an chemical compound with the chemical formula Cs2O. It is the simplest and most common oxide of the caesium. It forms yellow-orange hexagonal crystals.[1]


Caesium oxide is used in photocathodes to detect infrared signals in devices such as image intensifiers, vacuum photodiodes, photomultipliers, and TV camera tubes[3] L. R. Koller described the first modern photoemissive surface in 1929–1930 as a layer of caesium on a layer of caesium oxide on a layer of silver.[4] It is a good electron emitter; however, its high vapor pressure limits its usefulness.[5]


Elemental magnesium reduces caesium oxide to elemental caesium, forming magnesium oxide as a side-product:[6][7]

Cs2O + Mg → 2 Cs + MgO

Cs2O is hygroscopic, forming the corrosive CsOH on contact with water.


  1. ^ a b Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. pp. 451, 514. ISBN 0-8493-0487-3..
  2. ^ Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 97–100. ISBN 978-0-08-022057-4..
  3. ^ Capper, Peter; Elliott, C. T. (2000), Infrared Detectors and Emitters, Springer, p. 14, ISBN 978-0-7923-7206-6
  4. ^ Busch, Kenneth W.; Busch, Marianna A. (1990), Multielement Detection Systems for Spectrochemical Analysis, Wiley-Interscience, p. 12, ISBN 978-0-471-81974-5
  5. ^ Boolchand, Punit, ed. (2000), Insulating and Semiconducting Glasses, World Scientific, p. 855,, ISBN 978-981-02-3673-1
  6. ^ Turner Jr., Francis M., ed. (1920), The Condensed Chemical Dictionary, New York: Chemical Catalog Co., p. 121
  7. ^ Arora, M.G. (1997), S-Block Elements, New Delhi: Anmol Publications, p. 13, ISBN 978-81-7488-562-3